Electrostatic apparatus



Feb. 7, 1961 w. A. BRASTAD ELECTROSTATIC APPARATUS 3 Sheets-Sheet 1 Filed Sept. 4, 1956 FlG. 1

FIG. 2

IN VEN TOR.

[d/L L //7M /7. 52/75 7790 9M4; c. 6.1% ATTORNEY Feb. 7, 1961 w. A. BRASTAD ELECTROSTATIC APPARATUS 3 Sheets-Sheet 2 Filed Sept. 4, 1956 FIG. 3

INVENTOR.

ld/LL/HM fl 69/ 75740 BY M ATTORNEY "I I 'WI. mlmlh. ""HTI Feb. 7, 1961 w. A. BRASTAD ,971,127

' ELECTROSTATIC APPARATUS Filed Sept. 4, 1956 3 Sheets-Sheet 3 IN VEN TOR. dJ/L L //7/// ,4. 88 95 7740 ATTORNEY United States Patent ELECTROSTATIC APPARATUS William A. Brastad, Minneapolis, Minn., assignor to General Mills, Inc., a corporation of Delaware Filed Sept. 4, 1956, Ser. No. 607,869

10 Claims. (Cl. 317-9) The present invention relates to electrostatic apparatus and more particularly to an improved electrostatic system with a safety arrangement adapted to prevent explosions within the system.

Many forms of electrostatic apparatus are known in the prior art and have been designed to accomplish various types of operation. Such devices have been used, for example, as classifying or separating devices to segregate particular components of various mixtures. They have also been used on occasion as dust collectors or precipitating devices to clear the air of fine dirt or smoke particles.

In certain prior applications there is a possible danger of explosions, which might result if an explosive mixture, particularly a dust mixture, is processed in the space between electrodes which are maintained at a relatively high difference of potential. Particularly where the processing apparatus involves parallel electrode plates which may have substantial capacitance, there is a possibility of accumulating energy within the processing apparatus at a level suflicient to cause ignition of the particular mixture being treated.

With these problems of the prior art in view it is accordingly one object of the present invention to provide an improved electrostatic apparatus in which the possibility of explosions is minimized.

A further object is an electrostatic system in which the desired operating potentials may be safely maintained, while the total energy available in the system is kept below the energy level for ignition of the particular products to be processed.

Another object is an improved method of charging an electrostatic apparatus to the desired relative potential difference.

Still another object is a novel safety mechanism for incorporation between the power supply and charged members of an electrostatic processing unit.

Other objects and advantages will be apparent from the following specification in which certain preferred embodiments of the invention are described with particular reference to the accompanying drawings. In these drawings, wherein like reference characters indicate like parts,

Figure 1 is a schematic view of a complete electrostatic system embodying features of the present invention;

Fig. 2 is an enlarged side view of the improved safety mechanism included in the system of Fig. 1;

Fig. 3 is a partial sectional view on the line 33 of Fig. 2; Fig. 4 is a perspective view, with certain portions broken away and certain portions shown in section for clearness, of the safety mechanism of the preceding figures;

Fig. 5 is a view similar to Fig. 3 of an alternate embodiment of the invention adapted to subdivide the total capacitance of a number of processing machines;

Fig. 6 is a partial sectional view on the line 6-6 of Fig. 5; and

2,971,127 Patented Feb. 7, 1961 ice Fig. 7 is a schematic view similar to Fig. 1 showing a further modification of the invention.

In general, according to the present invention, it has been found that the above objectives can be accomplished by the incorporation into the desired electrostatic system of a novel method and apparatus for delivering the desired charges from a power supply to the processing unit. A preferred form of apparatus for this purpose is incorporated in the system shown generally in Figure 1.

Here, the power supply for the electrostatic processing apparatus is indicated generally at 10. The power supply is designed to provide the desired voltage for a proc essing unit indicated at 12. While the present invention can be used with other types of electrostatic apparatus,

the particular processing unit shown schematically at 12, by way of example, is essentially the same as that disclosed and claimed in Morrison, U.S.P. 2,615,570, which is assigned to the same assignee as the present application. Reference is made to the disclosure of that patent for such details of the particular apparatus as are not readily apparent from the schematic showing of Figure l.

The power supply 10 is provided with input leads 14 and 16 designed for connection to a suitable commercial current source such as the standard volt 60 cycle AC. supply lines which are available at most locations. The unit 10 involves a power pack of any suitable known construction designed to provide a relatively high potential D.C. difference at its output terminals 18 and 20. The actual voltage difference across these output terminals is shown by a suitable indicator 22 and this output voltage may be adjusted in known manner by a rheostat or control member 24. For most purposes, the potential difference between the output terminals 18 and 20 will be of the order of several thousand volts or more.

The electrostatic processing apparatus 12 includes a material input or supply chute 26 through which the desired material is fed to the space between lower and upper inclined substantially parallel electrodes 28 and 30 respectively. As the material passes along the upper surface of lower electrode 28 the apparatus is designed to subject the material to an electrostatic field which will attract certain portions toward or through the slotted upper electrode 30.

The desired potential difference between electrodes 28 and 38 is maintained by means of wire connections 32 and 34 which are adapted to connect these electrodes to the power supply terminals 18 and 20 through the safety mechanism to be described. Those particles which actually pass through the openings in upper electrode 30 may be separately collected at 36, while the portion remaining in the space between electrodes 28 and 30 may be further subdivided into a fraction collected at 38 and a tailings fraction conveyed by a chute 40 to a second pair of similar electrodes 42 and 44.

The upper electrode 42 of this second pair may be connected by wire 46 to the wire 32 by which the other upper electrode is to be charged, so that both electrodes may be charged to the same relative potentials at the same times. It will be understood that by suitable alternate arrangements, e.g. by the use of separate power packs, these electrodes could be charged separately to different potentials if desired for any particular operation. The lower electrode 44 of the second pair is similarly connected by wire 48 to the supply wire 34 for the lower electrode of the upper set.

The material which is attracted through the openings of the upper electrode 42 of the second set is separately collected and removed at 50, while the portion of material which remains in the field between the electrodes may be again subdivided as in the previous case into separate fractions collected at 52 and 54 respectively,

According to the present invention, a safety device 56 is connected in the system between the power supply terminals 18 and Zil on the one hand and the supply wires 32 and 34011 the "electrostatic.apparatus 12 on the other hand. This safety device 56 includes an output terminal 58 ;connectedtothe supply wire 32 for the upper electrodesof theapparatusand an. input :terminal 6% connected; by awire 62' to the high voltageterminal 1% of thenpQWersupply-dtl. The safety device also. includes a common. terminaled which, is connected to the wire 34 supplying,thelower: electrodes of the apparatus and is also connected byrawire 616 tothe lower voltage terminal 2d of the power supply. As indicated in Figure 1, theacommon terminal.64 may begrounded as at 68 where itcis.desiredthat.theparticular portions of the apparatus .connected by.this terminal be-maintained essentially at zero potential As shown in Figure 1 and-described above, the system is so connected thatone setofelectrodes, in this case the lower set 28 and. 44,will always be. connected at all timesto the groundedterminal ofthe power supply. The other electrodes 30 and 42-.of the apparatuswill be connected..::to the high voltage terminal 18 of the power supply in.the.manner and. by theapparatus features described below. This method andapparatus are particularly designed. to provide ,the desired. chargefor electrodes 30 and 42 without at any time making a total amount of energy available at the electrodes which would causea spark between thcelectrodes. under the most adverse operating conditions. contemplated. Specifically, the invention includesthe elimination of a complete direct circuit connectionxbetween the power source and load.

In this. regard, if the wire 32 were directly connected to. the output terminal 18 at all times, the desired voltage difference would be maintained between the respective pairs of electrodes, and these electrodes would also accumulateacharge depending on therelative capacitance between the electrodes. couldbe replenished at all timesby the direct connection to the power supply and since the power supply thus provides a direct source of considerable additional energy, there would be-substantial danger that in case ofa fault, the total energy available at the electrodes of the apparatus might be high enoughtocause a spark and cause ignition of the material being processed, particularly where such material is relatively fine and in cludes a substantial dust fraction.

As indicated, the spark or-arc may result from some fault in theapparatus. The breakage of a support might let one electrode touch-the other. Or a foreignsubstance suchas a piece of wire or screen might provide a discharge-point for such a spark.

The energy andduration of the sparkwill depend in largepart on the total {electrical capacity of the electrostatic apparatus-and associated elements. The ca- Since, however, this cl1argepacitance of separating devices which includes ,7 spaced parallel electrodes will'inturn 'dependon the area and spacing of the electrodes.- In the case of flour milling stocks, such as mixtures of particles of bran, particles of eudosperm, and/ or particles whichinclude both branaudendosperm in-varyingproportional have found that the danger of-ignitioncan be safely avoided, at electrostatic stocks. For other stocks, the energy levels above which ignition might occur can be determined as described below. 1

In some cases, to make sure the available energy levels are kept below the safe maximum, my invention contemplates the subdivision of the total load capacity by separate connection of individual units. Thus the capacitance of one of the separated units willnot be directly connected and available to support a spark in case of a fault in another unit.

According to a major feature of the present invention, the desired charge is transferred to the electrode supply wire 32 of the apparatus by a safety device which limits the maximum energy available at theapparatus to an amount below the minimum ignition energy of the particular material.

The illustrated preferred embodiment of this safety means includes what might be termed a distributor .56 provided with. therotary member WEI-(Fig. 4). This member is rotated'in the manner described below and carries one or more contact arms illustrated in this-case by the-two contactarms 72 and 74. Also carried by the rotary member-.70 are one or more condensers 76, 73, 80 and 32 whichare connected in circuit with the respective contact arms. Thus condensers .76 and 78 provide an efifective capacitance, one terminal of which isconnected to thecontact arm '72, while condensers 80 and 82 in this case form a second capacitance, one terminal or plate of which is effectively connected to the contact arm .74. The remaining plate or .terminal of each effective capacitance. unit is suitably connected to the. grounded terminal 64-.

These. condensers are designed to be connected. electri cally first to the supply terminal oil and grounded terminal 64, so that a charge will be acquired by the'condensers from the power unit lit. Next the condensers are designed to he moved out of electrical contact with the sup ply terminal 60.. The high potential plates of each condenser are next connected to. the terminal5i to which the supply wire 32'for the upper electrodes is connected. Thus the condensers successively acquire a charge. from the power pack and then they transfer the charge to the electrodes of the apparatus.- At the time the charge is being acquired, the condensers are out of circuit with the electrostatic apparatus. are in circuitwith theapparatusand are supplying a charge to it, they are out of circuit with. the powerunit. Therefore the power unit is unable to supply any further energy incase. of a fault, than that actually transferred by thecondensers themselves. The size of these condensers can be chosen in combination with the interconnected. load capacities so. that the total energy available for a possible spark is less than the energy level at Which ignition of the. particular product will occur,

To. support the transfer condensers, the rotary member I stationary distributor plate 1%. Plate 1% is preferably and preferably '(inyorder'to provide at least a 2 to :1

10,000 ;volts, thenthe total capacitance could be four times-thevalues specified above,- for theseflour milling madeof electrical insulating material. On the distributor plate llt o a plurality of contact platesare mounted. in the exampleshown in the drawings, there are two sets of these plates, with one set connected to the terminal cit fed'by' the power unit, and with the other set connected V to the output terminal 58- connected to the electrostatic apparatus. As illustrated, the first setjof plates 1th; includes specifically four plates-llil8A, 1MB, 168C and 108D. These plates are spaced equally around the cir- Sirnilarly,v. hen the condensers cumference of the distributor plate 106 so that they are 90 apart.

The second set of contact plates, including plates 110A, 110B, 110C and 110D, are also mounted at the periphery of the distributor plate 106 and are spaced equally from each other at points intermediate the first set of plates. The plate 108A of the first set includes terminal 60 connected by wire 62 to the power supply high voltage terminal 18 as in Figure 1. Wires 112, 114 and 116 then connect plate 108A with the other plates of the series as shown. Similarly, plate 110A is secured to distributor plate 106 by terminal screw 58 which is adapted to be connected to the supply wire 32 of the electrostatic apparatus. Plate 110A is then connected to the other plates of the second series by connecting wires 118, 120 and 122 as shown.

Since the contact plates of the 108 and 110 series are independently secured to and supported on the insulating distributor member 106, there is no direct electrical connection from one set of plates to the other. To carry the desired charges from the first set of plates 108 to the second set 110, the contact arms 72 and 74 of the rotary member 70 are designed to engage the plates of the respective series alternately and successively as the member 70 rotates in the direction of the arrows in Fig. 3. Thus contact arm 72 includes a substantially horizontal lower end 124 adapted to engage the different plates 108 and 110 successively with a wiping action. As illustrated, the plates are curved slightly so that their central portions are somewhat higher than the edges. Thus a smooth wiping action of the contact arm is assured.

The contact arm 72 has its upper end electrically and mechanically secured to a cnoducting strip 126 (Fig. 2) connected to one plate or terminal of the condenser 78. The other terminal of condenser 78 is connected by a wire 130 to the terminal of condenser 76. The remaining terminal of condenser 76 is then connected by conducting strip 132 (Fig. 4) to the metallic electrically-conducting bolt member 86 which secures the insulated rotary plate 70 to the metallic conducting flange 90 on the upper end of shaft 92.

Similarly, the contact arm 74 includes a substantially horizontal and slightly curved lower end 134 also adapted to engage the different plates 108 and 110 successively with a wiping action. The upper end of contact arm 74 is electrically and mechanically fastened to a conducting strip 136 connected to one terminal 138 of condenser 80. A connecting wire 140 joins the remaining terminal of condenser 82. The conducting strip 142 (Fig. 4) completes the circuit from condenser 82 through bolt 84 to the flange 90 and shaft portion 92.

A curved resilient metallic brush member 144 is carried by a suitable support 146 for constant wiping engagement with the shaft portion 92. The supporting strip 146 is in turn electrically connected to the terminal 64 which is the common terminal connecting wires 34 and 66 to ground wire 68.

The circuit arrangement just described provides two sets of condensers in each of which the two individual condensers are connected in series with each other to form an effective capacitance unit of the desired value. Where individual condensers with the desired values of capacitance and voltage'are readily available, only one condenser need be supplied for each desired contact arm. In the present case, in the example illustrated specifically below, the desired capacitance for each unit was 250 ,uptf. Because 500 ,lL/Lf. condensers were more readily and economically available, and in order to double the breakdown voltage of the transfer unit, two of these 500 f. condensers were connected in series as illustrated to provide an eifective capacitance of 250 /.,u.f. In cases where this capacitance is insuflicient to maintain the desired voltage of the load device against leakage currents, as discussed below, a single transfer condenser of 500 minis preferred.

While other arrangements of condensers may be utilized for specific applications, depending on the total capacitance desired, and on the size of available capacitors, the important fact is that the preferred embodiment of the invention provides a capacitance of appropriate size, which is alternately and successively connected, first across the power supply and then across the electrostatic apparatus. When the capacitance unit is transferring its charge to the apparatus there is no possibility of feeding to the apparatus any energy over and above the sum of that already present at the apparatus, plus that carried by the condenser unit of the distributor.

According to the embodiment shown in Figs. 5 and 6, a modified form of the invention is described which is designed primarily to subdivide the total load capacitance and permit separate charging of different portions of the load. Thus the total load capacitance available at any given time for a spark in one of the sections of the apparatus is reduced. Here there are two separate units 150 and 152 essentially similar to the two sections of the apparatus 12 of Fig. 1. The lower plate of each section 150 and 152 is grounded.

The upper section of apparatus 150 is electrically connected to plates 154 and 156 of a modified distributor 158, by Wires and 157. The upper electrode of apparatus section 152 is similarly connected to plates 160 and 162 of distributor 158 by wires 161 and 163 respectively. The remaining distributor plates 164, 166, 168 and 170 are connected to each other and to the high voltage side of the supply source by Wires 172, 174, 176 and 178. In other respects the apparatus is similar to that of the first embodiment.

With this arrangement of distributor plates and connections, the two contact arms of the distributor which are indicated at 124 and 134 respectively will transfer charges successively from the supply to one of the apparatus sections and then from the supply to the other apparatus section. The position of the contact arms is such that only one of them is connected to the supply at a time, while the other is discharging to its load section.

Also in this embodiment, the wires connecting the supply plates of the distributor are positioned on one side of the insulated distributor body portion 158, while the wires 157 and 163 are suitably spaced on the opposite side of the distributor body 158. Thus the supply wires do not at any time come in contact with the load wires connected to the apparatus. Any possibility of an are directly between the supply wires and the load Wires is accordingly avoided.

In the design of a suitable distributor mechanism according to the present invention, one other factor should be considered. No matter how well a given electrostatic processing apparatus is designed there may well be leakage effects which would cause the voltage between the electrodes to drop gradually when the electrodes were not connected to a supply source which would automatically replenish these losses. In most applications, the material between the electrodes may also provide a path through which charges may be transferred from one electrode to the other. Since the precision of operation of the device may depend on the maintenance of the voltage within a specified range, it is essential to provide a distributing unit which will transfer charges from the power supply to the apparatus rapidly enough to maintain the desired operatmg voltage. V

Ordinarily some deviation from the optimum operating voltage of the electrostatic apparatus can be tolerated and it is then possible to design and operate the distributor in a manner which on the one hand will supply charges rapidly enough to maintain the operating voltage within the permissible working range and at the same time, provide at any given instant a maximum energy level well below the minimum energy level for ignition of the particular material being processed. The rate of transfer can be controlled by adjustment of the rate of rotation 0t the distributor'plate '70, or by the choice ofa" suitable number of contact arm and condenser units,-or by theuse of higher capacitance in the transfer condenser, or by a combination of such factors.

As described above, by appropriate calculation (and subdivision, if necessary) of the capacitance of the electrodes in the operating unit, and by suitable choice of the size of condensers in the distributor unit, a construction can be achieved according to the present invention in which the maximum energy avail-able between the electrodes of the apparatus can always be kept below the minimum energy level at which the most explosive component of the material being treated will ignite.

There have been a number of studies of the energy levels required for ignition of particular mixtureswithin an electrostatic field. One such study by scientists of the Buread of Mines was reported in volume 15 of the Journal of Chemical Physics, at pp. 798802. in this study, sparks ofvariousenergy levels were discharged across a predetermined spark gap to determine the energy required to ignite the particular material. The size and type of spark gap may afiect the result as well as the type of material. In any event the minimum ignition energy may be considered as the lowest value of /2CV at which a spark can ignite the particular material at that gap. Here C is the total capacitance available and connected 'forpossible direct discharge across a particular gap, and V is the operating voltage to which the condense-rs are charged.

The minimum ignition energies for various materials can be determined experimentally by methods and ap paratus analogous to that described in the above paper; Also, the total energy available in a given electrostatic system accidentally to cause such ignition can be determined by computation from the voltage level desired and the capacitance of both the operating unit and the distributor illustrated in the present application. From these factors it is possible to check a given installation and to make certain that the capacitance values used are within a range which will limit the maximum available energy to a level well below the minimum ignition energy of the'product. Thus while the specific capacitance values required for the distributor of the present invention in combination with a given electrostatic system or apparatus willdepend on the nature and construction of the apparatus as Well as on the material to be processed by the apparatus, these values are capable of ready computation according to the principles set forth herein. By way of illustration of one form of apparatus in which specific operating voltages and capacitance values pro vide the desired safety factors, the following example is given.

A An electrostatic Figure l and shown in detail in USP. 2,615,570 was designed for classification of fineparticles such as those found in the middlings streams of a flour milling process. In this particular example the effective area of theeiectrode plateswas substantially L728 square inches and their relative spacing from each other was of the order of 0.5 inch. For the particular flour middlings stream to be classified, voltages of. the order of 7,000 volts per inch of separation of the electrodes were required. Thus in this apparatus the effective capacitance of the system was substantially 1,000 1 1f.

The specific safety mechanism designed for use in this system corresponded essentially to that shown in Figs.

'14 in which the oapacitance uni-t associated with each distributor arm included two condensers in series, each of'which had a ca acitahce of 500 n tf. Thus the effective capacitance for each distributor contact arm was 250 ##f. r i '0 In this example, with two contact arms and four contact plates in each of the sets 108 and 110, the rotary member was op'erated at an angular speed of 300 rpm; andthus transferred charges to the electrostatic apparatus at therate f 'ZAQO'char'ging operations per minute,

separator ofethe type illustrated in 8 taking into accouiit both contactar'rr'rsa'nd'both sets'of condensers.

The m'aximum'energy available between 'the'electrodes of the apparatus with this system was accordingly determined as follows by'adding the energy (one-half CV available through the capacitance of the electrodes to the corresponding energy available through the capacitance of the distributor:

W =total energy available at electrodes :1/2C'TVT2+%CEVE2 /2 (250x 10*) (4000) /2 (1000 10 (3500) =8:125 m-il'lijoules Where V =vol-tage of transfer condensers as charged by power supply=4000 volts V =voltage of electrodes just before contactby transfer condenser is made=3500 (V O -capacity of transfer condenser=250 tf. Cg=capacity of electrodes=1000 ,lLMf.

Thus the'maximum available energy of 8.125 joulesunder these operating conditions is well below the'safe flour milling limits set forth earlier in this description, where the range of 2900-3200 ,lL/.Lf. at 10,000 volts corresponds to energy levels from 145m 160 millijoules, and the preferred limit of 1 400 ,u tf. corresponds to 70 millijoules.

,At thesarne time, in thisparticula'r caseQit wa's'fourid thatunder normal operating conditions thevoltage drop at the electrodes of the apparatus between chargin'gs was- In the embodiment of the invention shown in Fig. 7,

thedistribtutor or safety device 56 is essentially similar to that of Figs. 1-4 except that each transfer capacitance includes a single 500 ,unf. condenser, as indicated schematically at 182 and 184. In this casethe capacitance of the load device, an electrostatic separator of the type described above, is preferably of theorder of 900 ,u if, for operation at voltages up to 10,000 volts. 7

According to a further modification of the invention, two resistors 186 and 188 are placed incircuit with the condensers of the transfer device. In this case each resistor has avalue of substantially 50,000 ohms. The insertion of these resistancesin the circuit with the transfer condensers substantially minimizes corrosioniof the contacts and brushes of the distributor. Thus the useful working life of the unit can be increased.

I lnthis enibodiment,as in the previous cases described, the distributor or protective device acts as a barrier which isolates the electrostatic power source .or supply from direct connection to the processing'unit o'rloadJ The device is arranged to'provide a means positively opening the circuit between the condenser or charge-carrying irn pedance and the load, during the time theconde'nser is connected to/the input terminals of the protective device, -i.e. the terminals adapted for connection to the power supply. Similarly, the relative arrangement, spacing and size of the parts provide means positively opening the circuit from the impedance to the supply, when the impedance is connected to the output leads for transfer of charge to the processing unit.

In this connection, the length of contact arms '01 brushes 72 and 74 (Fig. 2) must clearly be enough shorter than the space between contactplates 108' of the or output series to prevent arcing from one series to the other via the contact arm. The tops of all contact plates are carefully mounted in a common plane, and their convexly curved upper surfaces aid in providing a light brushing action for engagement and disengagement 'of the contact arms. Thus the alternate and repeated connection of the charge-carrying impedance to the supply and to the load can be accomplished with maximum electrical separation of the supply and load and with minimum Wear of the parts.

According to the foregoing description, a method and apparatus have been described which substantially accomplish the objectives set forth at the beginning of this application. According to the teachings of this invention, a safety means has been provided which isolates the load at all times from the power supply, which charges the load by means of a charging impedance connected alternately to the supply and load, and which at all times limits the maximum energy available between the electrodes of the electrostatic apparatus to a level well below the minimum energy level required for ignition of the particular material to be processed. Since minor variations and changes in the exact details of the method and apparatus will be apparent to persons skilled in this field, it is intended that this invention shall cover all such changes and modifications as fall within the spirit and scope of the attached claims.

Now, therefore, I claim:

1. An electrostatic protective device comprising a rotary member, a condenser carried by said member, a contact arm projecting from said member and electrically connected to one terminal of said condenser, a first series of contact plates spaced around said rotary member in position for successive engagement by said arm, first and second input terminals, means electrically connecting the first input terminal to each of said first series of contact plates, at least one second series of contact plates spaced around said rotary member with each plate of the second series located between plates of the first series for alternate engagement of said contact arm with successive plates of each series, first and second output leads, means electrically connecting the first output lead to each of said second series of contact plates, means electrically connecting the second input terminal and second output lead to the remaining terminal of the condenser, and driving means for rotating said rotary member and thereby engaging said contact arm successively and repeatedly with the alternate contact plates of the first and second series.

2. An electrostatic protective device according to claim 1 having a plurality of said contact arms spaced around said rotary member, and a plurality of said condensers at least one of which is connected with each arm, the relative angular spacing of said contact arms and contact plates placing one contact arm in contact with a plate of said first series while a second contact arm is in contact with a plate of said second series.

3. An electrostatic protective device according to claim 1 having a common supporting sheet of insulating matreial on which said first and second series of contact plates are mounted, the means electrically connecting the first input terminal to each plate of the first series being located entirely on one side of the supporting sheet, and the means electrically connecting the first output lead to each plate of the second series being located entirely on the opposite side of the supporting sheet.

4. An electrostatic device according to claim lhaving a third series of contact plates spaced around said rotary member with each plate of the third series located be tween plates of the first series, and with each plate of the third series separated from each plate of the second series by at least one intermediate plate of the first series, third and fourth output leads for another processing unit, means electrically connecting the third output lead to each plate of the third series, and means electrically connecting the fourth output lead to said second input terminal and second output lead.

5. An electrostatic protective device according to claim 1 having an electrical resistance connected in series with said condenser and contact arm during each engagement of the contact arm with said plates.

6. A device according to claim 5 in which said resistance includes one resistance connected in series with said first input terminal and first series of contact plates, and a second resistance connected in series with said first output terminal and second series of contact plates.

7. An electrostatic system comprising an electrostatic processing unit for flour milling stocks having a pair of electrodes and a predetermined electrical capacitance and adapted to operate at a predetermined voltage difference between said electrodes, an electrostatic power source for supplying at least the desired voltage difference, and a protective device for indirectly connecting said un.t to said source and including at least one electrical charge storing means having a predetermined electrical capacitance and means for alternately and repeatedly connecting said charge storing means first to said source and then to said unit, the total effective capacitance of said unit together with said charge storing means when the latter is connected to said unit having a value providing a maximum energy level for said predetermined operating voltage difference at all times less than substantially to millijoules.

8. An electrostatic flour milling system according to claim 7 in which said maximum energy level is substantially 70 millijoules.

9. An electrostatic flour milling system according to claim 8 in which the capacitance of said processing unit is of the order of 900 .t,uf., the capacitance of said charge storing means is substantially 500 fl, and the number of charge storing means and the relative frequency of repeated operation of said connecting means provides substantially 2400 charge transferring operations per minute at voltages not exceeding 10,000 volts.

10. An electrostatic protective device comprising a rotary member, a condenser carried by said member, a contact arm projecting from said member and electrically connected to one terminal of said condenser, first and second contact plates spaced around said rotary member in position for successive engagement by said arm, first and second input terminals, one of which is connected to said first contact plate and the other to the remaining terminal of said condenser, first and sec- 0nd output leads, means electrically connecting one output lead to the second contact plate and the other output lead to the remaining terminal of the condenser, and driving means for rotating said rotary member and thereby engaging said contact arm successively and repeatedly with the first and second plates.

References Cited in the file of this patent UNITED STATES PATENTS 2,381,250 Baumann Aug. 7, 1945 2,594,595 Stearns Apr. 29, 1952 2,807,011 Rowell Sept. 17, 1957 2,832,020 Towner Apr. 22, 1958 

